1. Field of the Invention
This invention relates to a method for applying polymer coatings to large substrate materials including steel, concrete, or wooden structures for protection against corrosion, weathering or other environmental damage. Included within the structures to which the method of this invention may be applied are buried steel pipelines used, for example, in the transmission and distribution of natural gas and oil. The method of this invention is particularly suitable for "in-the-field" applications and applications where maintaining the temperature of the substrate material to which the coatings are applied below a level at which the integrity of the substrate material is affected or at which a potentially hazardous condition is created is essential.
2. Description of Prior Art
Protective coatings are extensively used to protect metallic substrates, such as steel pipes and pipelines, from corrosion and mechanical damage. Widely used commercially available coatings for such substrates include fusion bonded epoxy (FBE) coatings. In the United States, FBE coatings are especially popular for pipeline protection because of their excellent anticorrosion properties, good adhesion to metal surfaces, and resistance to cathodic disbondment from the metallic substrate. However, when used alone, FBE coatings are prone to handling damage during pipe installation and also exhibit relatively high moisture permeation. Thus, most of the FBE coatings currently applied, especially in Europe, are an integral part of three-layer systems consisting of an epoxy (mainly FBE) primer, a plastic copolymer adhesive, and a plastic (polyolefin) outer sheath for protection of the epoxy primer. The basic principle in the three-layer systems is the use of an adhesive middle layer to provide the bonding agent between the epoxy primer and the plastic (polyolefin) outer layer. Polyolefins are preferred for use as a protective layer because they have many of the qualities lacking in isolated fusion bonded epoxy coatings, such as superior impact resistance, as well as improved impermeability to moisture and many chemicals. Polyolefins are also easy to fabricate for plant-applied coatings. However, because of their nonpolarity, polyolefins bond poorly with metallic substrates. Even the use of adhesives, such as copolymers, in bonding the polyolefin to the metallic substrate has not been found to provide a coating with equal properties to the epoxy/metal bond in terms of resistance to hot water immersion and cathodic disbandment. Another disadvantage of these systems, particularly when used in "in-the-field" applications on steel pipelines, is the time consuming preheat up to 450.degree. F. and the number of different materials and application means required for applying the coating layer. In "in-the-field" applications, it is highly desirable to minimize the amount of equipment and number of different materials to be applied.
Other coating systems known to afford protection against both corrosion and chemical attack include fluoroplastic coatings that afford excellent protection against chemicals and are not attacked by either strong acids or solvents. In addition to their well-known mechanical properties, such as high resistance to abrasion and good elasticity, the thermal properties of the fluoroplastics also allow them to be used just as they are, even when prolonged exposure to temperatures up to 260.degree. C. is involved. However, like other plastics, fluoroplastics exhibit both poor adhesion to steel surfaces and permeability to gases, liquids and solutions, thereby necessitating the application of relatively thick layers.
A process for powder coating high temperature resistant surfaces with multilayer coatings of fluoroplastics is taught by U.S. Pat. No. 4,999,221.
U.S. Pat. No. 4,510,007 teaches a method for jacketing steel pipes in which the pipe is heated to a temperature sufficiently to cause a subsequently applied epoxy resin-curing agent powder blend to melt after which a twin-foil of hose-like tubular configuration is extruded upon the precoated object under the proviso that the ethylene copolymer portion of the twin- or double-ply hose has been predried, and under the further assumption that the extrusion temperature particularly of the outer thermoplastic hose is in the range of about 165.degree. C. to 190.degree. C. Implementation of this method requires preheating the steel pipes to a temperature between about 175.degree. C. and 275.degree. C. in order to ensure melting of the powdered epoxy resin-curing agent powder blend. One problem with this method is that temperatures in the required range are difficult, if not impossible, to achieve on, for example, in situ underground pipelines. In addition, these temperatures are high enough that the integrity of any internal surface treatment, for example, internal pipeliners, could be compromised. U.S. Pat. No. 4,345,004 teaches a process for forming an olefinic resin film on a metal substrate comprising forming a multi-layer coated film consisting of an olefinic resin film as a surface layer portion and a cured epoxy resin film as an underlayer portion on a metallic substrate by a single coating operation using a multi-layer film-forming coating composition comprising as main resinous components a solid powder containing an olefinic resin having a melt index of 0.3 to 80 grams per 10 minutes, a solid powder containing a polar group-containing modified olefinic resin having a melt index of 0.3 to 80 grams per 10 minutes, and a film-forming resinous material comprising an epoxy resin having a number average molecular weight of about 350 to 4,000 and an epoxy equivalent of 150 to 3,800 and a curing agent therefor, and then heat-bonding an olefinic resin lining material to the olefinic resin surface layer of the multi-layer coated film.
See also U.S. Pat. No. 5,178,902 which teaches a method for applying and forming a protective composite coating on a metallic substrate in which the substrate is heated to a temperature between about 175.degree. C. and 275.degree. C. and a powdered coating of epoxy resin between 100 and 400 microns thick is applied to the outer surface of the heated substrate. A premixed powder coating of epoxy resin and polyolefin is applied directly onto the epoxy resin coating, forming an interlayer of interspersed domains of epoxy and polyolefin between about 100 and 400 microns in thickness. Onto this, powdered polyolefin is sprayed to produce a polyolefin sheath coating for the metallic substrate between 200 and 1,000 microns in thickness. In accordance with one embodiment, the interlayer is formed by spraying pure epoxy resin powder and polyolefin powder from separate sources simultaneously onto the substrate.
Application of a coating to a metallic substrate preheated to a temperature between about 160.degree. F. and about 240.degree. F. is frequently carried out by flame spraying in which a stream of pneumatically conveyed finely divided thermoplastic material is propelled through a flame and onto the substrate surface to be coated. The thermoplastic material becomes molten from the heat of the flame and is deposited onto the substrate surface where it cools and hardens to form a protective coating. Flame spray guns and processes employing flame spraying are well known in the art. See, for example, U.S. Pat. No. 5,211,990; U.S. Pat. No. 4,962,137; U.S. Pat. No. 5,041,713; U.S. Pat. No. 3,988,288; U.S. Pat. No. 4,985,278; and U.S. Pat. No. 4,276,390.
Russian Patent 407753 teaches a method for producing polymer coatings from powder thermoplastic materials in which a thermoset heat-resistant resin-based liquid primer is applied to a substrate and a thin layer of heated thermoplastic polymer powder is sprayed on the non-hardened sticky primer. After the primer has hardened at room temperature, the surface layer of the coating is heat treated, and additional layers of fused thermoplastic material are applied. A similar method for repair of pipelines is taught by U.S. Pat. No. 5,792,518. One problem associated with the methods taught by both of these patents arises from the requirement that the thin layer of thermoplastic polymer powder be applied to the primer layer before it has had an opportunity to harden. During the application of the thermoplastic polymer powder, it is not uncommon for the non-hardened primer surface to be breached by the pressurized spray resulting in the formation of air pockets within the primer which significantly reduce the overall strength and integrity of the primer layer. Another problem associated with the methods taught by these patents is the requirement that the thermoset heat resistant, resin-based primer harden at ambient temperature before the surface layer of the polymer powder coating can be applied, thereby rendering it unattractive for in-the-field use where it is undesirable to have workers unproductively waiting for the hardening to occur, which, at lower ambient temperatures, could be for extended periods of time. Still a further problem associated with the methods taught by these patents relates to the requirement that the thermoset heat-resistant, resin-based primer be applied at ambient temperatures as opposed to elevated temperatures. The flowability of the primer is substantially retarded at ambient temperatures rendering it difficult to apply evenly.
Yet another problem associated with conventional methods for coating steel, concrete, or wooden structures relates to the disposition of moisture between the structure and the coating. For example, underground pipelines, due to the temperature of the fluid flowing therethrough, generally "sweat" when exposed to ambient temperatures. Application of protective coatings by conventional means results in some water being trapped between the protective coating and the structure, thereby affecting in a negative way the integrity of the interface between the protective coating and the structure to be protected.